The present invention relates to a camera including a lens shutter and, more particularly, to a camera system including a focus detecting device which splits a photographing light beam before a lens shutter disposed in a photographing optical system and performs focus detection for the split light beam.
Generally, a camera system incorporating a lens shutter includes a distance measuring optical system in addition to a photographing optical system and performs focus detection by using an external distance measuring method such as an "external passive method". However, such an external distance measuring method cannot provide any satisfactory distance measurement accuracy because parallax is produced between the photographing optical system and the distance measuring optical system.
To solve this problem, therefore, Jpn. Pat. Appln. KOKAI Publication No. 9-218345 has proposed a camera which removes parallax by performing focus detection by splitting a light beam in the middle of the optical path of a photographing optical system and before a lens shutter and guiding the split light beam to focus detecting means.
In the invention disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-218345, the difference between the F number (the aperture F number during exposure) of a photographing light beam reaching the film surface and the aperture F number of a focusing light beam reaching the focus detecting means or the like factor changes the relative positional relationship between the best image plane of an object image formed by the light beam guided to the photographing optical system and the best image plane of an object image formed by the focusing light beam guided onto a photoelectric conversion element via a portion of the photographing optical system.
Unfortunately, Jpn. Pat. Appln. KOKAI Publication No. 9-218345 does not mention any means for correcting this change in the relative positional relationship between the two best image planes. Accordingly, these two best image planes change relative to each other by focusing, zooming, and stopping operations of the photographing optical system and different wavelength components of the photographing light beam. This degrades the focus detection accuracy to result in an out-of-focus state.